1. Field of the Invention
The present invention relates to an acoustic wave filter apparatus having a balance-unbalance conversion function. More specifically, the present invention relates to an acoustic wave filter apparatus having a structure in which first and second acoustic wave filter sections are located on one piezoelectric substrate, first balance terminals of the first and second acoustic wave filter sections are commonly coupled to each other, and second balance terminals thereof are commonly coupled to each other.
2. Description of the Related Art
In recent years, cell phones and the like have been required to include a filter apparatus having multiple passbands. As one example of this type of filter apparatus, the following Japanese Unexamined Patent Application Publication No. 2002-208832 discloses a surface acoustic wave filter apparatus shown in FIG. 16.
In a surface acoustic wave filter apparatus 1001 shown in FIG. 16, unbalance input terminals of first and second surface acoustic wave filter sections 1003 and 1004 are coupled to an unbalance terminal 1002. The surface acoustic wave filter sections 1003 and 1004 are surface acoustic wave filters each having an unbalance input terminal and first and second balance output terminals and having a balance-unbalance conversion function. First balance output terminals of the first and second surface acoustic wave filter sections 1003 and 1004 are commonly coupled to each other and then coupled to a first balance terminal 1005. Similarly, second balance output terminals thereof are commonly coupled to each other and then coupled to a second balance terminal 1006.
On the other hand, WO2006/003787 discloses a surface acoustic wave filter apparatus shown in FIG. 17. As shown in FIG. 17, in the surface acoustic wave filter apparatus, an illustrated electrode structure is formed on a piezoelectric substrate 1102. Here, a first surface acoustic wave filter section 1101 is coupled between an unbalance input terminal 1103 and first and second balance terminals 1104 and 1105. The first surface acoustic wave filter section 1101 includes three-IDT-type, longitudinally coupled resonator-type surface acoustic wave filter elements 1107 and 1108. Input terminals of the longitudinally coupled resonator-type surface acoustic wave filter elements 1107 and 1108 are commonly coupled to each other and then coupled to an unbalance terminal 1103 via a surface acoustic wave resonator 1106. An output terminal of the longitudinally coupled resonator-type surface acoustic wave filter element 1107 is coupled to the first balance terminal 1104 via a surface acoustic wave resonator 1109. Similarly, an output terminal of the longitudinally coupled resonator-type second surface acoustic wave filter element 1108 is coupled to the second balance terminal 1105 via a surface acoustic wave resonator 1110.
Also, a surface acoustic wave filter section 1101A, which is another surface acoustic wave filter section, is formed on the piezoelectric substrate 1102. The second surface acoustic wave filter section 1101A has the same configuration as that of the first surface acoustic wave filter section 1101. Therefore, corresponding portions of the second surface acoustic wave filter section 1101A are shown using reference numerals where A is added to each of the reference numerals of corresponding portions of the first surface acoustic wave filter section 1101. Thus, the description of the first surface acoustic wave filter section 1101 is used.
In the surface acoustic wave filter apparatus described in Japanese Unexamined Patent Application Publication No. 2002-208832, the first balance output terminals of the first and second surface acoustic wave filter sections 1003 and 1004 are commonly coupled to each other and the second balance output terminals thereof are commonly coupled to each other. In this configuration, a wiring line for commonly coupling the first balance output terminals and one for commonly coupling the second balance output terminals tend to be longer. This has caused a problem that the surface acoustic wave filter apparatus is affected by parasitic capacitances, inductances, or resistances of the wiring lines and thus the insertion loss is increased or the balancing is deteriorated.
Even if the surface acoustic wave filter apparatus described in WO2006/003787 is deformed and the first balance output terminals are commonly coupled to each other and the second balance output terminals are commonly coupled to each other as in the surface acoustic wave filter apparatus 1001 described in Japanese Unexamined Patent Application Publication No. 2002-208832, the surface acoustic wave filter apparatus is easily affected by parasitic capacitances, inductances, or resistances of the wiring lines as well. For this reason, the insertion loss may be increased or the balancing may be deteriorated as well.